Various methods for extracorporeal blood treatment or cleaning are used to remove substances usually eliminated with urine and for fluid withdrawal. In haemodialysis, the patient's blood is cleaned outside the body in a dialyser. The dialyser comprises a blood chamber and a dialysing fluid chamber, which are separated by a semipermeable membrane. During the treatment, the patient's blood flows through the blood chamber. In order to clean the blood effectively from substances usually eliminated with urine, fresh dialysing fluid flows continuously through the dialysing fluid chamber.
Whereas the transport of the lower-molecular substances through the membrane of the dialyser is essentially determined by the concentration differences (diffusion) between the dialysing fluid and the blood in the case of haemodialysis (HD), substances dissolved in the plasma water, in particular higher-molecular substances, are effectively removed by a high fluid flow (convection) through the membrane of the dialyser in the case of haemofiltration (HF). In haemofiltration, the dialyser functions as a filter. Haemodiafiltration (HDF) is a combination of the two processes.
In haemo(dia)filtration, a part of the fluid removed from the blood through the membrane of the dialyser is replaced by a sterile substitution fluid, which is generally fed to the extracorporeal blood circuit either upstream of the dialyser or downstream of the dialyser.
The supply of substitution fluid upstream of the dialyser is also referred to as pre-dilution and the supply downstream of the dialyser as post-dilution.
Apparatuses for haemo(dia)filtration are known, wherein the dialysing fluid is prepared online from fresh water and concentrations and the substitution fluid is prepared online from the dialysing fluid.
In the known haemo(dia)filtration apparatuses, the substitution fluid (substituate) is fed to the extracorporeal blood circuit from the fluid system of the machine via a substituate line. With pre-dilution, the substituate line leads to a connection point on the arterial blood line upstream of the dialyser or filter, whereas with post-dilution the substituate line leads to a connection point on the venous blood line downstream of the dialyser or filter.
With the known methods of chronic blood cleaning therapy, such as haemodialysis, haemofiltration and haemodiafiltration, the vascular connection (shunt) is generally applied between an artery and vein in operative surgery as an access to the patient's blood vessel system. When mention is made in the following of a “fistula”, this is understood to mean any kind of connection between a vein and an artery of the patient.
The blood flowing through the fistula is used only during the actual dialysis treatment. In the period free from dialysis, the blood flow in the fistula corresponds to a functional left/right shunt, wherein a part of the arterial blood is fed from the heart minute volume (HMV), bypassing a peripheral use, directly to the venous system and the heart. The fistula flow recirculates via the heart and lungs. The fractional part of the fistula flow in the heart minute volume is defined as the cardiopulmonary recirculation.
The cardiopulmonary recirculation not only has effects on the patient's circulatory load, but also on the efficiency of the dialysis. Since the dialysed blood from the extracorporeal circuit is mixed with the venous backflow from the large body circulation thereby bypassing the systemic circulatory areas, a systematic reduction in the concentration of the dialysable constituents in the arterial blood results (D. Schneditz et al., Cardiopulmonary recirculation during haemodialysis, Kidney Int. 42: 1450-1456, 1992).
Of importance for the functional capability of the fistula is its perfusion. If the fistula flow falls below a critical value, the risk of a fistula thrombosis then increases with the possible loss of the vascular access, which in dialysis treatment represents a considerable complication (W. Bay et al., Color Doppler flow predicts PTFE graft failure, J. Am. Soc. Nephrol. 5: 407 (1994)). If the fistula flow during the dialysis treatment is smaller than the extracorporeal blood flow (QB), local fistula recirculation occurs, wherein a fraction of the dialysed blood fed back to the fistula with the venous blood line is again fed to the dialyser via the arterial blood line. Fistula recirculation RA causes a significant reduction in the dialysis efficiency (F. Gotch, “Models to predict recirculation and its effects on treatment time in single-needle-dialysis,” First Intl. Symposium on Single-Needle-Dialysis, S. Rignoir, R. Vanholder and P. Ivanovich, Cleveland, ISAO Press, 1984, page 305 ff.). The measurement of the quality of the vascular access is therefore an important means of quality assurance in dialysis treatment.
International Patent Publication No. WO 98/32477 describes a method for measuring recirculation R, i.e., the sum of the fistula recirculation (RA) and the cardiopulmonary recirculation RCP. With the known method, a physical or chemical characteristic variable of the dialysing fluid is changed in the dialysing fluid path upstream of the dialyser, which leads to a change in the physical or chemical characteristic variable on the blood side. The change in the characteristic variable on the blood side leads to a change in the characteristic variable of the dialysing fluid downstream of the dialysing fluid chamber of the dialyser. In order to determine the recirculation, the characteristic variable in the dialysing fluid path downstream of the dialyser is measured and recirculation R is determined from the time-related course of the change in the characteristic variable. The dialysing fluid ion concentration, for example the Na concentration of the dialysing fluid, or the temperature of the dialysing fluid, can be changed and measured as a physical or chemical characteristic variable.
U.S. Pat. No. 5,830,365 describes a method for determining the cardiopulmonary recirculation, which is based on two measurements of the recirculation fraction following shortly after one another, which are carried out automatically before and after the reversal of the blood flow. The drawback is that the known method requires the reversal of the blood flow.
European Application No. EP 1 595 560 describes a method for determining the recirculation, wherein the viscosity of the blood in the arterial or venous blood line is changed by switching on and/or switching off the substituate pump. The change in the viscosity of the blood leads to a drop in pressure or rise in pressure on the venous or arterial side of the extracorporeal blood circuit, which is picked up in order to detect a recirculation on the basis of the change in pressure. The known method provides for a change in the substituate rate by switching on and/or switching off the substituate pump, whilst the blood pump in the extracorporeal blood circuit continues to convey blood at the preset blood flow rate.